1. Field of the Invention
The present invention relates to methods of preparing an Nb.sub.3 Al superconducting wire and an Nb.sub.3 Al superconducting stranded wire, and more particularly, it relates to methods of preparing an Nb.sub.3 Al superconducting wire and an Nb.sub.3 Al superconducting stranded wire serving as high magnetic field superconducting materials for superconducting magnets which can be employed for nuclear fusion or the like.
2. Description of the Background Art
An Nb.sub.3 Al superconducting material is expected particularly as a material for a nuclear fusion reactor which is exposed to high magnetic force in a high magnetic field, or a power storage superconducting material, due to a high critical current and excellent distortion resistance in a high magnetic field. However, it is difficult to work Nb.sub.3 Al, which is an intermetallic compound, into a thin wire due to inferior workability, dissimilarly to an alloy superconducting material such as an Nb-Ti material. In order to obtain an Nb.sub.3 Al superconducting wire, therefore, an Nb metal and an Al metal are generally composited with each other and then drawn into a thin wire, which in turn is finally heat treated.
Further, Nb.sub.3 Al is stable only under a high temperature at least 1600.degree. C. in a binary system of Nb-Al. In order to form Nb.sub.3 Al at a lower temperature, therefore, it is necessary to reduce distance of Nb and Al diffusion pairs below several 10 nm by a jelly roll method or the like.
In an Nb/Al composite material which is prepared by compositing Nb and Al metals, however, the filament is ununiformly deformed when the material is worked into a thin wire, due to inferior workability. Therefore, an Nb.sub.3 Al superconducting wire which is obtained by heat treating the composite material cannot attain a sufficiently high critical current density.
Further, a compound superconducting material such as Nb.sub.3 Al is coiled before a heat treatment by the so-called wind-and-react method, since its superconductivity is extremely deteriorated by distortion such as bending. When the coil as formed is increased in size, however, it is difficult to carry out a uniform heat treatment as a whole.
In order to improve superconductivity, it is preferable to form Nb.sub.3 Al which is close to a stoichiometric composition and provided with fine crystal grains by performing the heat treatment in a short time at the highest possible temperature, while maintaining this Nb.sub.3 Al by rapidly cooling the same to the room temperature. However, it is difficult to heat treat and cool a large-sized coil in such a manner.